The nineteenth century has been understood as an age in which states could wage war against each other if they deemed it politically necessary. According to this narrative, it was not until the establishment of the League of Nations, the Kellogg-Briand Pact, and the UN Charter that the 'free right to go to war' (liberum ius ad bellum) was gradually outlawed. Better times dawned as this anarchy of waging war ended, resulting in radical transformations of international law and politics. However, as a 'free right to go to war' has never been empirically proven, this story of progress is puzzling. In A Century of Anarchy?: War, Normativity, and the Birth of Modern International Order, Hendrik Simon challenges this narrative by outlining a genealogy of modern war justifications and drawing on scientific, political, and public discourses. He argues that liberum ius ad bellum is an invention created by realist legal scholars in Imperial Germany who argued against the mainstream of European liberalism and, paradoxically, that the now forgotten Sonderweg reading was universalized in international historiographies after the World Wars. A Century of Anarchy? is a compelling read for historians, jurists, political theorists, international relations scholars, and anyone interested in understanding the emergence of the modern international order. In this groundbreaking work, Simon not only artfully deconstructs the myth of liberum ius ad bellum but also traces the political and theoretical roots of the modern prohibition of war to the long nineteenth century (1789-1918).